Computer systems typically use a magnetic disk drive for memory storage. As is well known, a magnetic disk drive includes a rotating magnetic disk, a slider that has write and read heads, a suspension arm above the rotating disk and an actuator arm. As the magnetic disk rotates, the slider rides on a cushion of air over the surface of the magnetic disk and writes and reads data on selected tracks on the magnetic disk. The data is written and read from the magnetic disk as field signals using a write head and read head, respectively.
One type of magnetic sensor that is currently used as a read head is known as a “spin valve” sensor. A spin valve sensor is typically a sandwiched structure consisting of two ferromagnetic layers separated by a thin non-ferromagnetic layer. One of the ferromagnetic layers is called the “pinned layer” because it is magnetically pinned or oriented in a fixed and unchanging direction. The pinned layer is sometimes a laminate of two ferromagnetic layers that are separated by a coupling layer. Magnetic pinning of the pinned layer is frequently accomplished using an adjacent antiferromagnetic (AFM) layer, commonly referred to as the “pinning layer,” through exchange coupling. The pinning field generated by the antiferromagnetic layer should be greater than demagnetizing fields (about 200 Oe) at the operating temperature of the spin valve sensor (about 120° C.) to ensure that the magnetization direction of the pinned layer remains fixed during the application of external fields (e.g., fields from bits recorded on the disk).
The other ferromagnetic layer is referred to as the “free” or “unpinned” layer because the magnetization is allowed to rotate in response to the presence of external magnetic fields. The spin valve sensor provides an output which is dependent upon angle variation of the magnetizations between the free and pinned layers. Data recorded on a magnetic disk can be read because the external magnetic field (the signal field) causes a change in direction of magnetization in the free layer, which in turn causes a change in resistance of the spin valve and a corresponding change in the sensed current or voltage.
To achieve maximum amplitude linear response, the free and pinned layers of a spin valve must have well defined magnetization directions parallel and normal to the air bearing surface (ABS) of the slider, respectively. The magnetization of the free layer is typically set by abutted permanent magnets, which provide a longitudinal hard bias stabilization of the sensor parallel to the recording media surface. The magnetization of the pinned layer structure is conventionally established normal to the ABS during the antiferromagnetic anneal after the layers of the spin valve sensor have been deposited. The anneal “turns on” the exchange coupling between the antiferromagnetic pinning layer the pinned layer. The pinning direction of the pinned layer is determined by the direction of the magnetic field during cooling below the blocking temperature during the anneal process.
Another type of spin valve sensor uses a pinned layer that is self-pinned and therefore does not use an antiferromagnetic pinning layer. In a self-pinned sensor high coercivity or high uniaxial anisotropy due to magnetostriction of the pinned layer can be used to fix pinned layer magnetization normal to the ABS. Since self-pinned sensors do not have antiferromagnetic layers, some other means of setting the pinning direction of the pinned layer normal to the ABS is required.
The pinned layer structure in a self-pinned sensor can be comprised of a single high coercivity ferromagnetic layer, or in more advanced designs, a synthetic pinned layer is used comprised of two ferromagnetic layers, i.e. Keeper and Reference layers, which are antiferromagnetically coupled through a coupling or spacer layer. The coupling layer is usually comprised of Ru, but can be Ir, Rh, Os or their alloys. The ferromagnetic layer situated between the Cu spacer layer and the Ru spacer layer is referred to as the Reference layer. The other ferromagnetic layer situated between the Ru spacer layer and capping layer, or the Ru spacer layer and the seed layer is referred to as a Keeper layer in case of top and bottom spin valves, respectively. The thickness of the Ru spacer/coupling layer is selected to provide natural antiferromagnetic coupling between the spacer and Reference layers. The natural antiferromagnetic coupling strength has two maxima at about 4 Å Ru and 8 Å Ru, with coupling strength more than a factor of two higher for the case of 4 Å Ru. Thus, the effective coercivity or saturation field necessary to saturate the Keeper/Ru/Reference structure, is also more than a factor of two higher when the Ru thickness is approximately 4 Å. The high effective coercivity or saturation field of the Keeper/Ru/Reference structure is desirable to prevent performance degradation during sensor operation. It is also desirable for the pinned layer to have high intrinsic coercivity to prevent demagnetization during device fabrication or during sensor operation. A self-pinned sensor with a thin Ru spacer and high coercivity (>100 Oe) Keeper layer is referred to as coercivity pinned or hard pinned sensor to distinguish it from a magnetostriction pinned self-pinned sensor. The latter sensor's high effective magnetostriction of the Keeper/Ru/Reference structure, leads to a large magnetoelastic anisotropy in the lapped sensor, normal to the ABS.
When using a Ru spacer in the so called second peak, i.e., the Ru spacer is approximately 8 Å, a pinning direction normal to the ABS can be established by application of a large field after device fabrication. Magnetic fields typically achievable with conventional electromagnets, i.e., 2 Tesla or less, are adequate for this pinned layer setting operation. However in case of Ru spacers in the so called first peak, i.e., the Ru spacer is approximately 4 Å, the field required for magnetic saturation and setting of the pinned layer structure can exceed the maximum field provided by electromagnets. Thus some other means of establishing pinning direction are necessary in the case of the hard pinned sensor.
When available electromagnets do not allow pinned layer setting at the end of device fabrication, as it is often the case with hard pinned sensors, the pinned layers must be properly oriented during the deposition process itself, which can be accomplished by growing Keeper and Reference layers with well saturated magnetic moment in the as deposited state. The tools used to deposit magnetic sensors, allow application of a uniform magnetic field during deposition. This field is typically limited to about 100 Oe. In many systems, the application of the magnetic field during deposition causes an unacceptable non-uniformity in the thickness of the layers. Accordingly, the magnetic field applied during deposition is conventionally alternated in polarity at a large frequency, typically 25 Hz, i.e., an AC aligning field is used.
When growing a hard pinned structure the net coercivity of the deposited materials that evolves during the deposition process must be considered. In a “bottom” type spin valve, for example, the Keeper layer is deposited first. In some cases the Keeper layer has intrinsic coercivity below 100 Oe, so the alternating applied field can fully reverse its magnetization at every cycle. The same is true during the Ru spacer deposition. The Reference layer is deposited next and will orient itself anti-parallel to the Keeper layer. Due to this effect the net coercivity of the combined pinned layers will start to increase during the growth of the Reference layer. As the coercivity approaches 100 Oe, the applied field is no longer capable of saturating the pinned layer at each reversal and the pinned layer starts to demagnetize. As the coercivity increases further beyond 100 Oe, the pinned layers are permanently left in a partially demagnetized state, because the applied field is now too weak to affect the magnetization. As discussed above, even application of the largest fields from an electromagnet after deposition will not improve the pinned layer magnetic state in most cases with a thin Ru layer. Accordingly, for each hard pinned sensor, there is a critical thickness range in which the coercivity is comparable in size to the applied field. In cases where the Keeper layer coercivity is larger than 100 Oe, this critical thickness range occurs during deposition of the Keeper layer. The key to a well saturated pinned layer structure is, therefore, to carefully avoid field reversal during the critical thickness range. This constraint, in no way precludes achievement of an optimal thickness uniformity, as is discussed below.
In case of a thin Ru layer, e.g., approximately 4 Å, high coercivity or hard pinned self-pinned sensor, the use of an AC aligning field during deposition of the Keeper and Reference layers produces pinned layers whose magnetization is not saturated and whose field direction is hard to control. Accordingly, what is needed is an improved method of producing a pinned layer structure in a hard pinned self-pinned spin valve that provides a well defined magnetization state along with good thickness uniformity.